DevTools: [Styles] Do not bring up color picker for read-only style swatches

R=aandrey, eustas, vsevik
BUG=331056

Review URL: https://codereview.chromium.org/131613003

git-svn-id: svn://svn.chromium.org/blink/trunk@164862 bbb929c8-8fbe-4397-9dbb-9b2b20218538